Asteroids are rocky and metallic
objects that orbit the Sun but are too small to be
considered planets. They are known as minor planets.
Asteroids range in size from Ceres, which has a diameter of about
1000 km, down to the size of pebbles. Sixteen asteroids have a
diameter of 240 km or greater.
They have been found inside Earth's orbit to beyond Saturn's orbit. Most,
however, are contained within a main
belt that exists between the orbits of Mars and Jupiter.
Some have orbits that cross Earth's path and some have even hit the Earth
in times past. One of the best
preserved examples is
Barringer Meteor Crater near Winslow,
Arizona.

Asteroids are material left over from the formation of the
solar system. One theory suggests that they are the remains of a
planet that was destroyed in a massive collision long ago. More
likely, asteroids are material that never coalesced into a planet.
In fact, if the estimated total mass of all asteroids was gathered
into a single object, the object would be less than 1,500
kilometers (932 miles) across -- less than half the diameter of our
Moon.

Much of our understanding about asteroids comes from
examining pieces of space debris that fall to the surface of
Earth. Asteroids that are on a collision course with Earth are
called meteoroids.
When a meteoroid strikes our atmosphere at high
velocity, friction causes this chunk of space matter to incinerate
in a streak of light known as a meteor. If the meteoroid does not
burn up completely, what's left strikes Earth's surface and is
called a meteorite.

Of all the meteorites examined, 92.8 percent are composed of
silicate (stone), and 5.7 percent are composed of iron and nickel;
the rest are a mixture of the three materials. Stony meteorites
are the hardest to identify since they look very much like
terrestrial rocks.

Because asteroids are material from the very early solar
system, scientists are interested in their composition. Spacecraft
that have flown through the asteroid belt have found that the belt
is really quite empty and that asteroids are separated by very
large distances.
Before 1991
the only information obtained on asteroids was though Earth
based observations. Then on October 1991 asteroid
951 Gaspra was visited by the
Galileo spacecraft and became the
first asteroid to have hi-resolution images taken of it.
Again on August 1993 Galileo made a close encounter with
asteroid 243 Ida. This was the second
asteroid to be visited by spacecraft.
Both Gaspra and Ida are classified
as S-type asteroids composed of metal-rich silicates.

On June 27, 1997 the spacecraft NEAR
made a high-speed close encounter with asteroid
253 Mathilde. This encounter gave
scientists the first close-up look of a carbon rich C-type asteroid.
This visit was unique because NEAR was not designed
for flyby encounters. NEAR is an orbiter destined for asteroid
Eros in January of 1999.

Astronomers have studied a number of asteroids through Earth-based
observations. Several notable asteroids are
Toutatis,
Castalia,
Geographos and
Vesta.
Astronomers studied Toutatis, Geographos and Castalia using
Earth-based radar observations during close approaches to the Earth.
Vesta was observed by the Hubble Space Telescope.